1. Field of the Invention
The present invention relates to a power supply having a switchable polarity for use in an electrophotographic image forming apparatus. The present invention also relates to an image forming apparatus using such a power supply.
2. Description of the Related Art
For an electrophotographic image forming apparatus, as the size and weight of a high-voltage power supply for supplying a direct current bias to form an image in a transfer unit decrease, the size and weight of the image forming apparatus decreases. Accordingly, the transformer normally used for the high-voltage power supply, typically an electromagnetic winding transformer, is being replaced with a thin, light-weight high-power piezoelectric transformer. By using a ceramic-based piezoelectric transformer, a high voltage can be more efficiently generated compared with an electromagnetic transformer. In addition, the distance between a primary electrode and secondary electrode can be increased regardless of the coupling between the primary and secondary windings. This can eliminate the need for a special molding process for electrical insulation, thus providing a compact and light-weight high voltage power supply.
An exemplary circuit configuration of a high-voltage power supply with a piezoelectric transformer for outputting positive and negative polarities is now herein described with reference to FIG. 2. FIG. 2 illustrates a circuit of a high-voltage power supply used for attracting means that attracts a transfer medium onto a transport belt by charging the transfer medium in an image forming apparatus. This high-voltage power supply is an example of a high-voltage power supply for outputting positive and negative polarities.
The circuit mainly includes a positive voltage circuit section for generating an output voltage of positive polarity and a negative voltage circuit section for generating an output voltage of negative polarity. Basically, the negative voltage circuit section is similar to the positive voltage circuit section. By reversing a diode polarity of a diode rectifying section of a voltage output stage, the negative voltage circuit section generates a negative voltage.
The positive voltage circuit section includes an off-circuit section 220 in which a comparator 122 compares a voltage divided by resistors 123 and 124 with a positive voltage setting signal Vcont—+ so as to switch on and off 24-V power supply to an inductor 112 using a transistor 121. In consideration of a noise effect, the voltage divided by the resistors 123 and 124 is set to about 0.5 V. The supply to the inductor 112 is switched off when the positive voltage setting signal Vcont—+ is less than or equal to about 0.5 V. That is, when a negative voltage is output and a negative output detection signal Vsns is input to an operational amplifier of the positive voltage circuit section, power supply to a piezoelectric transformer 101 is stopped.
The configuration of the negative voltage circuit section is partially similar to that of the positive voltage circuit section. The similar components in the configuration of the negative voltage circuit section have the same reference numeral with a suffix “'” as those in the configuration of the positive voltage circuit section. Hereinafter, only the positive voltage circuit section is described. The positive voltage circuit section includes the high-voltage piezoelectric transformer (piezoelectric ceramic transformer) 101. The output from the piezoelectric transformer 101 is rectified to a positive voltage and is smoothed by diodes 102 and 103 and a high-voltage capacitor 104. The output is supplied to an attracting roller 500 (see FIG. 3), which is a load of the circuit. In an output voltage detection circuit 206, the output voltage is divided by resistors 105, 106, and 107. The divided voltage is input to a non-inverting input terminal (“+” terminal) of an operational amplifier 109 via a protective resistor 108.
Here, the output voltage detection circuit 206 is configured as shown in FIG. 2 using the resistors 105, 106, and 107 and a capacitor 115 so as to function as a filter circuit. Accordingly, the output voltage detection signal Vsns is input to the operational amplifier 109 according to a circuit time constant determined by the part constants of the resistors and capacitors. In contrast, the positive output voltage setting signal Vcont—+ of the high-voltage power supply, which is an analog signal from a DC controller 201, is input to an inverting input terminal (“−” terminal) of the operational amplifier 109 via a resistor 114. Here, the operational amplifier 109, the resistor 114, and a capacitor 113 are configured as shown in FIG. 2 so as to function as an integration circuit. The integration circuit has an integration time constant determined by the part constants of the resistors and capacitors. An output terminal of the operational amplifier 109 is connected to a voltage controlled oscillator (VCO) 110. An output terminal of the VCO 110 controls a transistor 111 connected to the inductor 112 so that power is supplied to a primary side of the piezoelectric transformer.
In general, as shown in FIG. 5, a piezoelectric transformer has a mountain-shaped output characteristic with respect to frequency, in which an output voltage becomes maximum at a resonant frequency of f0. Accordingly, by changing a driving frequency, the output voltage can be controlled. For example, by changing the driving frequency from a frequency sufficiently higher than the resonant frequency of f0 to a lower frequency (but still higher than the resonant frequency of f0), the output voltage of the piezoelectric transformer can be increased. Such a high-voltage power supply with a piezoelectric transformer is disclosed in, for example, Japanese Patent Laid-Open No. 11-206113.
When this known example is applied to a high-voltage power supply that requires a positive polarity output and a negative polarity output, the following problems occur.
In general, when the output is supplied to attracting means, a positive polarity output is applied at moments when a transfer medium is passing over the attracting means. In contrast, a negative polarity output that is the same polarity as the toner is applied at moments when there is no sheet passing over the attracting means (i.e., in an intersheet gap) in order to prevent toner from being adhered to the surface of the attracting roller, which is the attracting means, and from contaminating the surface. Therefore, as the state changes from a sheet-passing state to an intersheet gap, and subsequently to a sheet-passing state, the attraction bias output needs to change from positive to negative, and subsequently to positive. For example, in an image forming apparatus having a process speed of about 120 mm/sec, to print about 21 A4-pages per minute, the intersheet gap time is about 400 msec.
An example of a positive/negative output switching control is described next with reference to FIGS. 2 and 7.
In a circuit shown in FIG. 2, the integration circuit constant of an input stage for receiving the positive output voltage setting signal Vcont—+ is determined by the 1 MΩ resistor 114 and the 4700 pF capacitor 113. The circuit outputs a positive output voltage of about 1.1 kV DC according to a positive output voltage setting signal Vcont—+ of about 5 V. The circuit then changes the output to a negative output voltage of about −500 V DC, and then changes the output to a positive output voltage of about 1.1 kV DC again. The control of this change is described below.
In FIG. 7, the abscissa represents the time and the ordinate represents the voltage. The positive/negative output voltage setting signal Vcont is shown in the lower blocks. The output signal is shown in the upper blocks.
Firstly, a positive output voltage setting signal Vcont—+ is turned off to turn off the positive output voltage to 0 V. Subsequently, a negative output voltage setting signal Vcont—− is turned on so as to apply a negative pre-bias voltage for about 300 msec. A negative target voltage of −500 V is then output. Thereafter, to switch the negative output to a positive output, the negative output voltage setting signal Vcont—− is turned off so as to turn off the negative output voltage to 0 V. Subsequently, the positive output voltage setting signal Vcont—+ is turned on so as to apply a positive pre-bias voltage for about 300 msec. A positive target voltage of +1.1 kV is then output. Thus, in a known switching control, as shown in FIG. 7, a control is performed as follows: outputting a positive voltage, turning off the positive output, outputting 0 V, applying a negative pre-bias, outputting a negative voltage, turning off the negative output, outputting 0 V, applying a positive pre-bias, and outputting a positive voltage. That is, the output is turned off so that the output once becomes 0 V. A pre-bias voltage is then applied to raise the next output voltage. To perform this control, the pre-bias is applied for about 300 msec so that a switching time of about 1000 msec is required. This relatively long switching time does not allow the switching to be completed within the intersheet gap time.